General capacitors have a structure in which a dielectric is interposed between a pair of electronically conductive electrodes. Capacitors are energy storage devices in which charges are accumulated in proportion to a voltage applied between a pair of electrodes.
Of these capacitors, supercapacitors show battery characteristics and superior discharge characteristics, including instantaneous high current and high power, despite low energy density as compared to common batteries. In addition, supercapacitors can withstand several hundreds of thousands of charge/discharge cycles, indicating a feasible semi-permanent cycle life. Based on these advantages, extensive research and development have been conducted on supercapacitors.
Supercapacitors are divided into the following types: (1) electric double layer capacitors for energy storage using an anode and a cathode, both of which are made of activated carbon, to form an electric double layer; and (2) hybrid batteries for energy storage using a first electrode made of a material capable of storing charges by electrical redox reactions and a second electrode made of an electric double layer capacitor material.
Since the electric double layer charge storage in the second electrode takes place by physical adsorption and desorption of ions arising from an electric potential, the reaction rate is very high and the charge/discharge cycle life is significantly extended while the storage capacity (or energy density) is lowered.
On the other hand, since the charge storage in the first electrode takes place in a bulk portion rather than on surfaces by electrical redox reactions, the reaction rate is low in materials for secondary batteries but the energy storage capacity is 10 times higher than that in the electric double layer charge storage.
The hybrid batteries are energy storage devices that use a material for a secondary battery as a material for the first electrode and an electric double layer capacitor material as a material for the second electrode. Accordingly, such constitution of the hybrid batteries overcomes low capacity, which is a disadvantage of electric double layer capacitors, and short cycle life and low power density, which are disadvantages of secondary batteries. According to the state of the art, hybrid batteries have about two-fold higher energy density than electric double layer capacitors, and ensure a cycle life of 10,000 cycles or more.
The hybrid batteries are largely classified into the following systems: (1) systems that use an anode made of a metal oxide material, e.g., lithium manganate, for a secondary battery, and a cathode made of activated carbon, which is used in electric double layer capacitors; (2) systems that use an anode made of activated carbon and a cathode made of graphite or mesocarbon microbeads (MCMB), which are used in secondary batteries. The systems (2) in which activated carbon is used as a material for an anode and graphite is used as a material for a cathode, are currently being developed for many applications because they have a working voltage of a maximum of 4 V and exhibit excellent cycle characteristics and high-temperature characteristics. However, the systems (2) have a problem in that a large excess of lithium must be added to activate the cathode, making it difficult to manufacture the systems (2) on an industrial scale.
In the systems (1) in which lithium manganate is used as a material for an anode and activated carbon is used as a material for a cathode, the use of an aprotic solvent, such as acetonitrile (AN) or propylene carbonate (PC), as a solvent for an electrolyte is under consideration, and mixed salts of tetraethylammonium tetrafluoroborate (TEABF4) and lithium tetrafluoroborate (LiBF4), etc. are used as salts of the electrolyte. Particularly, only one salt is not used, but two salts are used in hybrid batteries. The reason for the use of two salts, such as lithium tetrafluoroborate and tetraethylammonium tetrafluoroborate, in hybrid batteries is that energy is stored in an anode by intercalation/deintercalation of Li ions (Li+) while energy is stored in a cathode by adsorption/desorption of TEA ions (TEA+).
FIG. 1 is a graph showing the conditions of a cycle test for a hybrid battery specified by the US Department of Energy (DOE), and FIG. 2 is a graph showing changes in capacity according to the kind of electrolytes used in hybrid batteries under the test conditions shown in FIG. 1.
With reference to FIG. 1, in accordance with the procedure proposed by the US DOE, a cycle test for a hybrid battery is conducted by continuously repeating a cycle consisting of charging from ½ Vw (working voltage) to Vw for 20 seconds at a charge/discharge current of 50 mA/F, maintaining at Vw for 10 seconds, discharging from Vw to ½ Vw for 20 seconds, and maintaining at ½ Vw for 10 seconds.
In FIG. 2, for example, the cycle life of hybrid batteries is measured at a working voltage (Vw) of 2.5 V and a charge/discharge current of 50 mA/F. Specifically, the cycle life is measured under the output conditions by continuously repeating a cycle (60 seconds) consisting of charging from 1.25 V to 2.5 V for 20 seconds, maintaining at 2.5 V for 10 seconds, discharging from 2.5 V to 1.25 V for 20 seconds, and maintaining at 1.25 V for 10 seconds.
The graph of FIG. 2 compares the use of propylene carbonate with the use of acetonitrile as a solvent of an electrolyte under the test conditions shown in FIG. 1. For the experiments of FIG. 2, LiMn2O4 was used as a material for an anode and activated carbon was used as a material for a cathode to manufacture capacitors. 1 M lithium tetrafluoroborate (LiBF4) and 1 M tetraethylammonium tetrafluoroborate (C2H5)4NBF4) were used as solutes of an electrolyte when acetonitrile was used as a solvent of the electrolyte, while 0.75 M lithium tetrafluoroborate (LiBF4) 0.75 M and 0.75 tetraethylammonium tetrafluoroborate (C2H5)4NBF4) were used as solutes of an electrolyte when propylene carbonate was used as a solvent of the electrolyte.
The graph of FIG. 2 shows that about 90% of the initial capacity was maintained after 10,000 cycles when acetonitrile was used as a solvent of an electrolyte, whereas only 70% or less of the initial capacity was maintained after 10,000 cycles when propylene carbonate was used as a solvent of an electrolyte.
This difference in the decrease in capacity is attributed to the fact that the carbonate material has a low electrical conductivity, a low solubility for the salts and a high possibility of redox reactions at high temperatures when compared to acetonitrile.
Acetonitrile can be used to manufacture high-power hybrid batteries because of its low viscosity and high solubility for salts. Accordingly, acetonitrile is suitable for use as a solvent of an electrolyte. However, acetonitrile has a low boiling point of about 82° C., is highly flammable, and has a high probability of forming cyanide when a fire occurs. Particularly, when it is intended to design large-scale products, heating to 140° C. or higher results in sublimation of electrolytes present in the products, thus risking the danger of sudden explosion. In addition, acetonitrile is an organic cyanide compound classified into categories of toxic substances, and therefore, there is a limitation in use from a standpoint of technical design valuing environmental stability.
Propylene carbonate is widely used as a solvent of an electrolyte due to its non-toxicity, safety and high boiling point. However, propylene carbonate has a higher resistance and a lower solubility for salts than acetonitrile. Accordingly, there is a limitation in using propylene carbonate in the manufacture of large-sized products requiring high power and low resistance.
Thus, there is a need for a hybrid battery using a carbonate-based solvent, which is more stable than acetonitrile, as a solvent of an electrolyte, thereby achieving improved high-power and cycle characteristics.